The pyrolytic and kinetic characteristics of a macroalga (Sargassum thunbergii) and a freshwater plant (Potamogeton crispus) were evaluated and compared at heating rates of 10, 30 and 50 degrees C min (1) under N-2 atmosphere using a thermalgravimetry (TG) and the evolved gases were analyzed online using a mass spectroscopy (MS). Three stages appeared during the pyrolysis process; dehydration, primary devolatilization and residual decomposition. Significant differences in average activation energy, thermal stability, final residuals and reaction rate existed between P. crispus and S. thunbergii. For P. crispus, the most probable mechanism was represented by a three dimensional diffusion model, whereas it was interpreted as random nucleation and nuclei growth for S. thunbergii. There were kinetic compensation effects between the pre-exponential factors and the activation energy. There was H-2 evolved during the pyrolysis of P. crispus, but no obvious H2 detected for that of S. thunbergii. For CO, CO2, H2O, C1-C3 alkanes, their evolve stage were corresponding mainly with the weight loss stage. For C4-C7 alkenes and aromatic compounds, however, its evolution peak temperature zone was gradually far from the second weight loss peak temperature zone. (C) 2012 Elsevier Ltd. All rights reserved.